In a radial tire for aircraft, an extremely-high specified internal pressure exceeding 10 atmospheric pressures is defined by an official standard, but also tire reinforcing members are required to have a pressure resistance corresponding to four times of the specified internal pressure. Further, exacting requirements for the safety of the tire are imposed, and hence it is required to provide a durability capable of running safely even if the tire treads foreign matters on an airstrip or a heavy load exceeding an usual use condition of tire is applied.
In order to meet these requirements, there is proposed a tire structure for preventing invasion of foreign matters into a tread portion of a tire at a state of inflating under a high internal pressure (WO 2003/061991). At this moment, it is possible to provide an excellent durability against the foreign matters by applying a high-strength and high-elastic modulus cord to a belt of a tire to suppress an extension of a tire tread in a circumferential direction in the filling of an internal pressure. However, when such a high-elastic modulus member is used in the belt cord, there are technical problems in tire manufacturing steps, particularly a vulcanization step of applying high temperature/high pressure to a green tire.
The vulcanization step of the tire is to promote a cross-linking reaction of rubber constituting the tire as well as to bond an interface between rubber members laminated in the tire building. In the tread portion of the tire, the adhesiveness of the rubber interface particularly plays an important role in the durability of the tread portion. In the tire vulcanization step, since a green tire is usually formed so as to be smaller as compared with a vulcanization mold, it is common that the green tire is somewhat enlarged by applying a high pressure to the green tire between a vulcanization bladder and the mold in the vulcanization. A ratio of a green tire enlarged during the vulcanization is referred to as a tire enlargement ratio, which is an important parameter in terms of manufacture. As requirements for ensuring the adhesion between rubber members are generally mentioned the following two points: (1) a sufficient vulcanizing pressure and (2) a rubber flow (at interface between members) during vulcanization. It is known that if these factors are lacking, the adhesion between rubber members is deteriorated. As to the rubber flow of the above item (2), there is the conventional knowledge that the larger the tire enlargement ratio and hence tire extension ratio during vulcanization, the larger the rubber flow.
Since the tire described in WO 2003/061991 uses a high-elastic modulus cord as compared with a previous radial tire for aircraft, if the enlargement ratio is made equal to that of the conventional tire, a part of pressure applied by the bladder during the vulcanization is born by the enlarged belt cord, and hence a pressure applied to the rubber interface in the tread portion is lowered and a desired adhesion is not obtained, so that it is necessary to set the enlargement ratio so as to be small. On the other hand, the rubber flow tends to be reduced by the lowering of the enlargement ratio. In order to establish these conflicting factors, a strict control of the enlargement ratio is demanded, which hinders the improvement of productivity. Even in terms of tire performances, such a control similarly becomes an obstruction in the attempt of further improving a high-speed durability of the tread.
Heretofore, as disclosed in Japanese Patent Application No. S63-307691, a protection belt layer of the tire is constituted by forming a high-tenacity organic fiber cord as typified by an aromatic polyamide (commonly known as an aramid fiber) into a corrugated shape and winding a great number of such cords in a circumferential direction, which plays a role for preventing the invasion of foreign matters. The conventional organic fiber cords do not cause the cord shrinkage in the heating or the change of cord length in the vulcanization of the tire, so that they have no effect of improving the vulcanizing pressure or rubber flow.